The present invention generally relates to compound semiconductor devices, and in particular to compound semiconductor devices such as a high electron mobility transistor and a metal semiconductor field effect transistor. More particularly, the present invention relates to compound semiconductor devices having nonalloyed ohmic contacts.
At present, the practical use of integrated circuit devices utilizing compound semiconductor devices is advanced. Such integrated circuit devices have a good potential for use in future because of their excellent physical properties. Further, improvements of integration level, speeding-up and lower consumed power are desired.
Currently, electronic circuits fabricated as a compound semiconductor integrated circuit device are logic circuits. In general, a unit cell or circuit of a logic circuit is an inverter which consists of a driving-side-transistor and a load-side-transistor. In many cases, each transistor employed in the compound semiconductor integrated circuit device is a high electron mobility transistor (hereafter simply referred to as HEMT) or a metal semiconductor field effect transistor (hereafter simply referred to as MESFET).
As is well known, both alloyed ohmic contacts and Schottky contacts are widely employed for aluminum gallium arsenide/gallium arsenide (AlGaAs/GaAs) HEMTs, GaAs MESFETs and the like. Ohmic contacts are used for source and drain electrodes, and Schottky contacts are used for gate electrodes. The source and drain electrodes are generally made of AuGe, which is subjected to an alloying process to make ohmic contacts with source and drain regions. Schottky gate electrodes are generally made of Al or other metals having high melting points, and makes the Schottky contact with channels. It is noted that the alloyed source and drain electrodes differ from the Schottky gate electrodes in used material. An example of a conventional HEMT having the above configuration is disclosed in U.S. Pat. No. 4,424,525, for example.
The above fact that the alloyed source and drain electrodes and Schottky gate electrodes are made of mutually different materials is disadvantages for realizing compound semiconductor integrated circuits of a higher integration level. This is because contact regions are necessary to connect the source or drain electrode to the Schottky gate electrodes to configure a desired logic circuit. The presence of the contact regions prevents improvements in the integration level. That is, an increased area of the contact regions on a semiconductor chip is needed as the integration level increases. Hence, the possible integration level of the compound semiconductor integrated circuit is unsatisfactory at present.
Further, it is noted that an alloying process for forming ohmic contacts is complex and cumbersome.